During flight, the control of aerodynamic vehicles, such as aircraft, is principally accomplished via a variety of flight control effectors. These flight control effectors include aerodynamic controls such as the rudder, elevators, ailerons, speed brakes, engine thrust variations, nozzle vectoring and the like. By altering the various flight control effectors, the system state vector that defines the current state of the aerodynamic vehicle may be changed. In this regard, the system state vector of an aerodynamic vehicle in flight typically defines a plurality of current vehicle states such as the angle of attack, the angle of side slip, the air speed, the vehicle attitude and the like.
Historically, the flight control effectors were directly linked to various input devices operated by the pilot. For example, flight control effectors have been linked via cabling to the pedals and the control column or stick. More recently, the flight control effectors have been driven by a flight control computer which, in turn, receives inputs from the various input devices operated by the pilot. By appropriately adjusting the input devices, a pilot may therefore controllably alter the time rate of change of the current system state vector of the aerodynamic vehicle.
Actuation of the various control effectors may excite one or more dynamic structural modes of an aerodynamic vehicle. For an aircraft having a plurality of actuators disposed along the trailing edge of a wing, deflection of the actuators may create a bending moment on the wing. In this instance, the bending moment is an example of a dynamic structural mode excited by activation of the actuators. Other examples of the dynamic structural modes excited by the actuation of various control effectors of an aerodynamic vehicle include fuselage bending due to lateral loads from the vertical stabilizers.
The excitation of a dynamic structural mode may influence the control sensors that provide feedback to the flight control system. This feedback may, in turn, lead to unstable structural dynamics which may, in turn, lead to overloading of the control actuators and/or excessive structural fatigue. This phenomena is commonly termed control structure interaction (CSI).
Historically, CSI has been problematic in the development of control systems for large space structures and aerodynamic vehicles, such as commercial and military aircraft. For example, certain unmanned air vehicle development programs have required redesign of their control systems based upon CSI issues. Since the excitation of dynamic structural modes and the corresponding issues with CSI are only created by control inputs having frequencies at or near the frequencies of the structural modes, conventional techniques for addressing CSI have followed one of two paths. In one approach, the aerodynamic vehicle is designed such that the modal frequencies of the various dynamic structural modes that could otherwise be excited by actuation of the control effectors have modal frequencies that are outside the control bandwidth. In other words, the modal frequencies of the structural modes of the aerodynamic vehicle are offset from the frequency of the control inputs. In order to design an aerodynamic vehicle having modal frequencies that are outside the control bandwidth, however, the aerodynamic vehicle must generally be made quite rigid and stiff. Unfortunately, the design of a relatively rigid and stiff aerodynamic vehicle typically leads to increased weight and is oftentimes impractical.
A second approach is to utilize frequency dependent filters. These frequency dependent filters may be positioned so as to filter the signals monitored by the various control sensors such that the control sensors are not influenced by the excitation of the dynamic structural modes. Additionally or alternatively, the frequency-dependent filters may be positioned to filter the actuator commands to the control effectors such that the commands received by the various control effectors do not include frequency components at or near the modal frequencies of the various structural modes. While such filtering may at least reduce CSI issues, this filtering correspondingly reduces the performance of the flight control system by blinding the flight control system to some of the signals otherwise received by the control sensors and/or by blocking at least some of the inputs to the various control effectors. According to either approach, aerodynamic vehicles are produced that may cost more to design, but that do not perform as well.
It would therefore be desirable to avoid CSI issues that might otherwise overload the control effectors and/or may create excessive structural fatigue on an aerodynamic vehicle. However, it would be desirable to address CSI issues in a manner that does not increase the cost of an aerodynamic vehicle and does not reduce the performance of the aerodynamic vehicle.